LM7705MME/NOPB

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+ In - In shutdown low voltage amplifier true zero output voltage-V -0.23V C F+ VSS SD VDD VSS VOUT C RES C F- LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 LM7705LowNoiseNegativeBiasGenerator Check forSamples: LM7705 1FEATURES DESCRIPTION The LM7705 isa switchedcapacitorvoltageinverter 2• RegulatedOutput Voltage−0.232V with a low noise,−0.23V fixednegativevoltage• Output VoltageTolerance5% regulator.Thisdeviceisdesignedtobe used withlow

Output VoltageRipple4 mV PP voltageamplifiersto enable the amplifiersoutputto swingtozerovolts.The −0.23voltsisused tosupply• Max Output Current26 mA the negative supply pin of an amplifierwhile• Supply Voltage3V to5.25V maintaininglessthen 5.5 voltsacrossthe amplifier.
Conversion Efficiencyup to98% Rail-to-Railoutputamplifierscannotoutputzerovolts when operatingfroma singlesupplyvoltageand can• QuiescentCurrent78 µA resultin erroraccumulationdue to amplifieroutput• Shutdown Current20 nA saturationvoltagebeing amplifiedby followinggain• Turn on Time 500 µs stages.A smallnegativesupplyvoltagewillprevent theamplifiersoutputfromsaturatingatzerovoltsand• OperatingTemperature Range −40°C to125°C willhelpmaintainan accuratezero througha signal• 8-PinVSSOP Package processingchain.Additionally,when an amplifieris used to drivean ADC ’s input,itcan outputa zeroAPPLICATIONS voltagesignaland thefullinputrangeofan ADC can
True Zero AmplifierOutput be used. The LM7705 has a shutdown pin to minimizestandbypower consumption• PortableInstrumentation
Low VoltageSplitPower Supplies TypicalApplication These deviceshave limitedbuilt-inESD protection.The leadsshouldbe shortedtogetherorthedeviceplacedinconductivefoam duringstorageorhandlingtopreventelectrostaticdamage totheMOS gates. Pleasebe aware thatan importantnoticeconcerningavailability,standardwarranty,and use incriticalapplicationsof Texas Instrumentssemiconductorproductsand disclaimerstheretoappearsattheend ofthisdatasheet. 2Alltrademarksarethepropertyoftheirrespectiveowners. PRODUCTION DATA informationiscurrentas ofpublicationdate. Copyright© 2008–2013,Texas InstrumentsIncorporatedProductsconform to specificationsper the terms of the Texas Instrumentsstandardwarranty.Productionprocessingdoes not necessarilyincludetestingofallparameters.

SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com ABSOLUTE MAXIMUM RATINGS (1)(2) VALUE SupplyVoltage +5.75V VDD -VSS SD VDD +0.3V,VSS -0.3V Forinputpinsonly 2000V Human Body Model Forallotherpins 2000V ESD Tolerance(3) Machine Model 200V Charged DeviceModel 750V StorageTemp. Range −65°C to150°C JunctionTemperature(4) 150°C max MountingTemperature InfraredorConvection(20sec) 260°C (1) AbsoluteMaximum Ratingsindicatelimitsbeyond whichdamage tothedevicemay occur.OperatingRatingsindicateconditionsfor whichthedeviceisintendedtobe functional,butspecificperformanceisnotensured.Forensuredspecificationsand testconditions, see theElectricalCharacteristics. (2) IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheTISalesOffice/Distributorsforavailabilityand specifications. (3) Human Body Model,applicablestd.MIL-STD-883,Method 3015.7.Machine model,applicablestdJESD22 –A115–A (ESSD MM srdof JEDEC). FieldinducedCharge-DeviceModel,applicablestd.JESD22 –C101 –C. (ESD FICDM stdofJEDEC). (4) Typicalvaluesrepresentthemost likelyparametricnorm as determinedatthetimeofcharacterization.Actualtypicalvaluesmay vary overtimeand willalsodepend on theapplicationand configuration.The typicalvaluesarenottestedand arenotensuredon shipped productionmaterial. OperatingRatings SupplyVoltage(VDD toGND) 3V to5.25V SupplyVoltage(VDD wrtVOUT ) 3.23Vto5.48V TemperatureRange −40°C to125°C ThermalResistance(θJA ) 8-PinVSSOP 253°C/W 3.3VElectricalCharacteristics Unlessotherwisespecified,alllimitsareensuredforTA = 25°C, VDD = 3.3V,VSS = 0V,SD = 0V,C FLY = 5 µF,C RES = 22 µF, C OUT = 22 µF.Boldfacelimitsapplyattemperatureextremes(1). Symbol Parameter Conditions Min (2) Typical(3) Max (2) Units VOUT OutputVoltage IOUT = 0 mA −0.242 −0.232 −0.219 −0.251 −0.209 V IOUT = −20 mA −0.242 −0.226 −0.219 −0.251 −0.209 VR OutputVoltageRipple IOUT = −20 mA 4 mV PP IS SupplyCurrent No Load 50 78 100 μA150 ISD Shutdown SupplyCurrent SD = VDD 20 nA ηPOWER CurrentConversionEfficiency −5 mA ≤ IOUT ≤ −20 mA 98 % ηPOWER CurrentConversionEfficiency IOUT = −5 mA 98 % tON TurnOn Time IOUT = −5 mA 500 μs tOFF TurnOffTime IOUT = −5 mA 700 μs tOFF CP TurnOffTime Charge Pump IOUT = −5 mA 11 μs ZOUT OutputImpedance −1 mA ≤ IOUT ≤ −20 mA 0.23 0.8 Ω1.3 IO_MAX Maximum OutputCurrent VOUT < −200 mV -26 mA fOSC OscillatorFrequency 92 kHz (1) Boldfacelimitsapplytotemperaturerangeof−40°C to125°C (2) Alllimitsarespecifiedby testingorstatisticalanalysis. (3) Typicalvaluesrepresentthemost likelyparametricnorm as determinedatthetimeofcharacterization.Actualtypicalvaluesmay vary overtimeand willalsodepend on theapplicationand configuration.The typicalvaluesarenottestedand arenotspecifiedon shipped productionmaterial.

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www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 3.3VElectricalCharacteristics(continued) Unlessotherwisespecified,alllimitsareensuredforTA = 25°C, VDD = 3.3V,VSS = 0V,SD = 0V,C FLY = 5 µF,C RES = 22 µF, C OUT = 22 µF.Boldfacelimitsapplyattemperatureextremes(1). Symbol Parameter Conditions Min (2) Typical(3) Max (2) Units VIL Shutdown InputLow 1.6 V1.25 VIH Shutdown InputHigh 1.85 V2.15 IC Shutdown PinInputCurrent SD = VDD 50 pA Load Regulation 0 mA ≤ IOUT ≤ −20 mA 0.12 0.6 %/mA0.85 5.0VElectricalCharacteristics Unlessotherwisespecified,alllimitsareensuredforTA = 25°C, VDD = 5.0V,VSS = 0V,SD = 0V,C FLY = 5 µF,C RES = 22 µF, C OUT = 22 µF.Boldfacelimitsapplyattemperatureextremes(1). Symbol Parameter Conditions Min (2) Typical(3) Max (2) Units VOUT OutputVoltage IOUT = 0 mA −0.242 −0.233 −0.219 −0.251 −0.209 V IOUT = −20 mA −0.242 −0.226 −0.219 −0.251 −0.209 VR OutputVoltageRipple IOUT = −20 mA 4 mV PP IS SupplyCurrent No Load 60 103 135 μA240 ISD Shutdown SupplyCurrent SD = VDD 20 nA ηPOWER CurrentConversionEfficiency −5 mA ≤ IOUT ≤ −20 mA 98 % ηPOWER CurrentConversionEfficiency IOUT = −5 mA 98 % tON TurnOn Time IOUT = −5 mA 200 μs tOFF TurnOffTime IOUT = −5 mA 700 μs tOFF CP TurnOffTime Charge Pump IOUT = −5 mA 11 μs ZOUT OutputImpedance −1 mA ≤ IOUT ≤ −20 mA 0.26 0.8 Ω1.3 IO_MAX Maximum OutputCurrent VOUT < − 200 mV −35 mA fOSC OscillatorFrequency 91 kHz VIL Shutdown InputLow 2.55 V1.95 VIH Shutdown InputHigh 2.8 V3.25 IC Shutdown PinInputCurrent SD = VDD 50 pA Load Regulation 0 mA ≤ IOUT ≤ −20 mA 0.14 0.6 %/mA0.85 (1) Boldfacelimitsapplytotemperaturerangeof−40°C to125°C (2) Alllimitsarespecifiedby testingorstatisticalanalysis. (3) Typicalvaluesrepresentthemost likelyparametricnorm as determinedatthetimeofcharacterization.Actualtypicalvaluesmay vary overtimeand willalsodepend on theapplicationand configuration.The typicalvaluesarenottestedand arenotspecifiedon shipped productionmaterial. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 3 ProductFolderLinks:LM7705

VCP,IN fosc VOUT VCP,OUT POST REGULATOR C out VDD VSS VSS VREF2 4 5 LM7705 LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com Connection Diagram 8-PinVSSOP -Top View PIN DESCRIPTIONS Pin Number Symbol Description

1 C F+ C FLY PositiveCapacitorConnection

2 VSS Power Ground

3 SD Shutdown Pin

IfSD pinisLOW, deviceisON IfSD pinisHIGH, deviceisOFF

4 VDD PositiveSupplyVoltage

5 VSS Power Ground

6 VOUT OutputVoltage

7 C RES ReserveCapacitorConnection

8 C F- C FLY NegativeCapacitorConnection

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TEMPERATURE (° C) OUTPUT VOLTAGE RIPPLE (mV PP ) -40 0 40 80 120 C RES = CFILTER = 22 éF C RES = CFILTER = 10 éF SUPPLY VOLTAGE = 3.3V TEMPERATURE (° C) OUTPUT VOLTAGE RIPPLE (mV PP ) -40 0 40 80 120 C RES = CFILTER = 22 éF C RES = CFILTER = 10 éF SUPPLY VOLTAGE = 5.0V OUTPUT CURRENT (mA) OUTPUT VOLTAGE (V) -0.20 -0.21 -0.22 -0.23 -0.24 -0.25 0 5 10 15 20 25 30 -40° C 125° C 25° C 85° C SUPPLY VOLTAGE = 3.3V OUTPUT CURRENT (mA) OUTPUT VOLTAGE (V) -0.20 -0.21 -0.22 -0.23 -0.24 -0.25 0 10 20 30 40 50 60 -40° C 125° C 25° C 85° C SUPPLY VOLTAGE = 5.0V SUPPLY VOLTAGE (V) OUTPUT VOLTAGE (V) -0.19 -0.20 -0.21 -0.22 -0.23 -0.24 IOUT = 0 mA IOUT = 5 mA IOUT =10 mAIOUT = 20 mA SUPPLY VOLTAGE (V) SUPPLY CURRENT ( éA) 300 250 200 150 100 -40° C 125° C 25° C 85° C LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 TypicalPerformance Characteristics VDD = 3.3Vand TA = 25°C unlessotherwisenoted. Output Voltage Supply Current vs. vs. Supply Voltage Supply Voltage Figure1. Figure2. Output Voltage Output Voltage vs. vs. Output Current Output Current Figure3. Figure4. Output VoltageRipple Output VoltageRipple vs. vs. Temperature Temperature Figure5. Figure6. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 5 ProductFolderLinks:LM7705

TURN ON TIME (200 és/DIV) OUTPUT VOLTAGE (0.2V/DIV) ENABLE PULSE 0 mA 5 mA 10 mA 20 mA SUPPLY VOLTAGE = 3.3V ENABLE VOLTAGE TURN ON TIME (100 és/DIV) OUTPUT VOLTAGE (0.2V/DIV) ENABLE PULSE 0 mA 5 mA 10 mA 20 mA SUPPLY VOLTAGE = 5.0V OUTPUT CURRENT (mA) CURRENT CONVERSION EFFICIECY (%) 110 105 100 0 4 8 12 16 20 -40° C 125° C 25° C 85° C SUPPLY VOLTAGE = 3.3V OUTPUT CURRENT (mA) CURRENT CONVERSION EFFICIECY (%) 110 105 100 0 4 8 12 16 20 -40° C 125° C25° C 85° C SUPPLY VOLTAGE = 5.0V OUTPUT CURRENT (mA) SUPPLY CURRENT (mA) 0 4 8 12 16 20 -40° C 125° C 25° C 85° C SUPPLY VOLTAGE = 3.3V OUTPUT CURRENT (mA) SUPPLY CURRENT (mA) 0 4 8 12 16 20 -40° C 125° C 25° C 85° C SUPPLY VOLTAGE = 5.0V LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com TypicalPerformance Characteristics(continued) VDD = 3.3Vand TA = 25°C unlessotherwisenoted. Supply Current Supply Current vs. vs. Output Current Output Current Figure7. Figure8. CurrentConversion Efficiency CurrentConversion Efficiency vs. vs. Output Current Output Current Figure9. Figure10. Turn On Time Turn On Time Figure11. Figure12.

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OUTPUT CURRENT (mA) TIME (20 us/DIV) OUTPUT VOLTAGE (V) -0.210 -0.218 -0.226 -0.234 -0.242 -0.250 -10 OUTPUT CURRENT +25° C +85/+125° C -40° C SUPPLY VOLTAGE = 3.3V OUTPUT CURRENT (mA) TIME (20 us/DIV) OUTPUT VOLTAGE (V) -0.210 -0.218 -0.226 -0.234 -0.242 -0.250 -10 OUTPUT CURRENT +25° C +85/+125° C -40° C SUPPLY VOLTAGE = 5V OUTPUT CURRENT (mA) TIME (20 us/DIV) OUTPUT VOLTAGE (V) -0.210 -0.218 -0.226 -0.234 -0.242 -0.250 -10 OUTPUT CURRENT +25° C +85/+125° C -40° C SUPPLY VOLTAGE = 3.3V OUTPUT CURRENT (mA) TIME (20 us/DIV) OUTPUT VOLTAGE (V) -0.210 -0.218 -0.226 -0.234 -0.242 -0.250 -10 OUTPUT CURRENT +25° C +85/+125° C -40° C SUPPLY VOLTAGE = 5V TEMPERATURE (° C) LOAD REGULATION (%/mA) 0.4 0.3 0.2 0.1 0.0 -40 0 40 80 120 SUPPLY VOLTAGE = 3.3V TEMPERATURE (° C) LOAD REGULATION (%/mA) 0.4 0.3 0.2 0.1 0.0 -40 0 40 80 120 SUPPLY VOLTAGE = 5.0V LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 TypicalPerformance Characteristics(continued) VDD = 3.3Vand TA = 25°C unlessotherwisenoted. Load Regulation Load Regulation vs. vs. Temperature Temperature Figure13. Figure14. TransientResponse TransientResponse Figure15. Figure16. TransientResponse TransientResponse Figure17. Figure18. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 7 ProductFolderLinks:LM7705

TEMPERATURE (° C) OSCILLATOR FREQUENCY (kHz) 100 -40 0 40 80 120 SUPPLY VOLTAGE = 3.3V SUPPLY VOLTAGE = 5V SHUTDOWN VOLTAGE (V) OUTPUT VOLTAGE (V) -0.05 -0.10 -0.15 -0.20 -0.25 0 1 2 3 4 5 SUPPLY VOLTAGE = 5V SUPPLY VOLTAGE = 3.3V SHUTDOWN VOLTAGE (V) SUPPLY CURRENT ( éA) 300 250 200 150 100 0 1 2 3 4 5 SUPPLY VOLTAGE = 5V SUPPLY VOLTAGE = 3.3V LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com TypicalPerformance Characteristics(continued) VDD = 3.3Vand TA = 25°C unlessotherwisenoted. Output voltage Supply Current vs. vs. shutdown Voltage Shutdown Voltage Figure19. Figure20. OscillatorFrequency vs. Temperature Figure21.

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OUT=V- C RES CAP + CAP - Ó1 Ó2 LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013

APPLICATION INFORMATION

Thisapplicationssectionwillgivea descriptionof the functionalityof the LM7705. The LM7705 isa switched capacitorvoltageinverterwitha low noise,−0.23V fixednegativebiasoutput.The partwilloperateovera supply voltagerangeof3 to5.25Volt.Applyinga logicallow leveltotheSD inputwillactivatethepart,and generatea fixed−0.23V outputvoltage.The partcan be disabled;the outputisswitchedto ground level,by applyinga logicalhighleveltotheSD inputofthepart. FUNCTIONAL DESCRIPTION The LM7705, low noisenegativebiasgenerator,can be used formany applicationsrequiringa fixednegative voltage.A key applicationfortheLM7705 isan amplifierwitha truezerooutputvoltageusingtheoriginalparts, whilenotexceedingthemaximum supplyvoltageratingsoftheamplifier. The voltageinversionin the LM7705 is achievedusing a switchedcapacitortechniquewithtwo external capacitors(CFLY and C RES ).An internaloscillatorand a switchingnetworktransferscharge between the two storagecapacitors.ThisswitchedcapacitortechniqueisgiveninFigure22. Figure22. VoltageInverter The internaloscillatorgeneratestwo anti-phaseclocksignals.Clock 1 controlsswitchesS1 and S2. Clock 2 controlsswitchesS3 and S4. When SwitchesS1 and S2 are closed,capacitorC FLY ischarged to V+. When switchesS3 and S4 are closed(S1 and S2 are open) chargefrom C FLY istransferredtoC RES and theoutput voltageOUT isequalto-V+. Due totheswitchedcapacitortechniquea smallripplewillbe presentattheoutputvoltage,witha frequencyof the oscillator.The magnitudeof thisripplewillincreaseforincreasingoutputcurrents.The magnitudeof the ripplecan be influencedby changingthevaluesoftheused capacitors. Inthenextsectiona more detailedtechnicaldescriptionoftheLM7705 willbe given. TECHNICAL DESCRIPTION As indicatedin the functionaldescriptionsection,the main functionof the LM7705 isto supplya stabilized negativebiasvoltageto a load,usingonlya positivesupplyvoltage.A generalblockdiagram forthischarge pump inverterisgiveninFigure23.The externalpower supplyand loadareadded inthisdiagramas well. Figure23. LM7705 Architecture The architecturegiveninFigure23 shows thattheLM7705 contains3 functionalblocks:

Pre-regulator
Charge pump inverter
Post-regulator The outputvoltageisstabilizedby: Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 9 ProductFolderLinks:LM7705

I = f Âq = f CFLY (V1 ± V2) Âq = q1 -q2 = CFLY (V1 ± V2) V1 V2 C RES R LC FLY B A C FLY CHARGE PUMP INVERTOR C RESERVE VREF1 PRE REGULATOR VCP,IN fosc VOUT VCP,OUT POST REGULATOR C out VDD VSS VSS VREF2 LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com

Controllingthepower suppliedfromthepower supplytothechargepump inputby thepre-regulator
The power suppliedfromthechargepump outputtotheloadby thepost-regulator. A more detailedblockdiagram of the negativebiasgeneratorisgiveninFigure24. The controlof the pre- regulatorisbased on measuringthe outputvoltageof the charge pump. The goalof the post-regulatoristo providean accuratecontrollednegativevoltageattheoutput,and actsas a lowpass filtertoattenuatetheoutput voltageripple.The voltagerippleisa resultoftheswitchingbehaviorofthechargepump and isdependentofthe outputcurrentand thevaluesoftheused capacitors. Figure24. Charge Pump InverterwithInput/OutputControl In the nextsectiona simpleequationwillbe derived,thatshows the relationbetween the rippleof the output current,thefrequencyoftheinternalclockgeneratorand thevalueofthecapacitorplacedattheoutputofthe LM7705. Charge Pump Theory Thissectionuses a simplifiedbut realisticequivalentcircuitthatrepresentsthe basicfunctionof the charge pump. The schematicisgiveninFigure25. Figure25. Charge Pump When theswitchisinpositionA, capacitorC FLY willchargetovoltageV1.The totalchargeon capacitorC FLY is Q 1 = C FLY x V 1.The switchthenmoves topositionB, dischargingC FLY tovoltageV2.Afterthisdischarge,the charge on C FLY willbe Q 2 = C FLY x V2. Note thatthe charge has been transferredfrom the sourceV1 to the outputV2.The amount ofchargetransferredis: (1) When theswitchchanges between A and B ata frequencyf,thechargetransferperunittime,orcurrentis: (2) The switchedcapacitornetworkcan be replacedby an equivalentresistor,as indicatedinFigure26.

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R EQ = 1 f CFLY ¹ I = R EQ1 f CFLY ¹ V1 ± V2 V1 ± V2= V1 V2 R L R EQ C RES LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 Figure26. Switched CapacitorEquivalentCircuit The valueof thisresistorisdependent on both the capacitorvalueand the switchingfrequencyas givenin Equation3 (3) The valueforR EQ can be calculatedfromEquation3 and isgiveninEquation4 (4) Equation4 show thatthevaluefortheresistanceatan increasedinternalswitchingfrequency,allowsa lower valuefortheused capacitor. Key Specification The key specificationsfortheLM7705 aregiveninthefollowingoverview: SupplyVoltage The LM7705 willoperateover a supplyvoltagerange of 3V to 5.25V,and meet the specificationsgivenin the ElectricalTable.Supply voltagelower than3.3Voltwilldecreaseperformance(The outputvoltagewillshifttowards zero,and the currentsinkcapabilitieswilldecrease)A voltagehigherthan 5.25Vwillexceed theAbs Max ratingsand thereforedamage thepart. OutputVoltage/LineRegulation The fixedand regulatedoutputvoltageof −0.23 V has tightlimits,as indicatedin the ElectricalCharacteristicstable,to ensure a stablevoltage level.The usage ofthepre-and postregulatorincombinationwiththecharge pump inverterensuresgood lineregulationof0.29%/V Outputcurrent/Loadregulation The LM7705 can sinkcurrents> 26 mA, causingan outputvoltageshiftto −200 mV. A specifiedload-regulationof0.14% mA/V ensuresa minorvoltage deviationforloadcurrentup to20 mA. Quiescentcurrent The LM7705 consumes a quiescentcurrentlessthan100 µA.Sinkinga load current,willresultina currentconversionefficiencybetterthan90%, even for loadcurrentsof1 mA, increasingto98% fora currentof5mA. In the nextsectiona generalamplifierapplicationrequiringa true-zerooutput,willbe discussed,showing an increasedperformanceusingtheLM7705. GENERAL AMPLIFIER APPLICATION Thissectionwilldiscussa generalDC coupledamplifierapplication.First,one ofthelimitationsofa DC coupled amplifierisdiscussed.Thisisillustratedwithtwo applicationexamples.A solutionisa givenforsolvingthis limitationby usingtheLM7705. Due tothearchitectureoftheoutputstageofgeneralamplifiers,theoutputtransistorswillsaturate.As a result, theoutputofa generalpurposeop amp can onlyswing toa few 100 mV ofthesupplyrails.Amplifiersusing CMOS technologydo have a lower outputsaturationvoltage.This is illustratedin Figure27. E.g.Texas Instruments'LM7332 can swingto200 mV tothenegativerail,fora 10 kΩ load,overalltemperatures. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 11 ProductFolderLinks:LM7705

GAIN = 50x ADC122S021LMP7701 SENSOR VIN VOUT VDSAT INPUT VOLTAGE (V) OUTPUT VOLTAGE (V) 0 V+ VDSAT OUTPUT SATURATION LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com Figure27. LimitationoftheOutput ofan Amplifier The introductionofoperationalamplifierswithoutputRail-to-raildrivecapabilitiesisa strongimprovementand the (output)performanceof op amps isformany applicationsno longera limitingfactor.For example,Texas Instruments'LMP7701 (a typicalrail-to-railop amp), has an outputdrivecapabilityof only 50 mV over all temperaturesfora 10 kΩ loadresistance.Thisisclosetothelowersupplyvoltagerail. However,fortruezerooutputapplicationswitha singlesupply,thesaturationvoltageoftheoutputstageisstilla limitingfactor.Thislimitationhas a negativeimpacton thefunctionalityoftruezerooutputapplications.Thisis illustratedinFigure28. Figure28. Output LimitationforSingleSupply True Zero Output Aapplication Inthefollowingsection,two applicationswillbe discussed,showing thelimitationsoftheoutputstageofan op amp ina singlesupplyconfiguration.

A singlestagetruezeroamplifier,witha 12 bitADC back end.
A dualstagetruezeroamplifier,witha 12 bitADC back end. One-stage,SingleSupply True Zero Amplifier Thisapplicationshows a sensorwitha DC outputsignal,amplifiedby a singlesupplyop amp. The outputvoltage oftheop amp isconvertedtothedigitaldomain usingan AnalogtoDigitalConverter(ADC).Figure29 shows the basicsetupofthisapplication. Figure29. Sensor withDC Output and a SingleSupply Op Amp

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GAIN = 10x GAIN = 5x ADC122S0211/2 LMP7702 1/2 LMP7702 SENSOR LM7705 www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 The sensorhas a DC outputsignalthatisamplifiedby the op amp. For an optimalsignal-to-noiseratio,the outputvoltageswing of the op amp shouldbe matched to the inputvoltagerange of the Analog to Digital Converter(ADC).For thehighsideoftherangethiscan be done by adjustingthegainoftheop amp. However, thelowsideoftherangecan’tbe adjustedand isaffectedby theoutputswingoftheop amp. Example: Assume the outputvoltagerange of the sensoris0 to 90 mV. The availableop amp isa LMP7701, usinga 0/+5V supplyvoltage,havingan outputdriveof50 mV frombothrails.Thisresultsinan outputrangeof50 mV to4.95V. Letchoose two resistorsvaluesforR G1 and R F1 thatresultina gainof50x.The outputoftheLMP7701 should swingfrom0 mV to4.5V.The highervalueisno problem,however thelowerswingislimitedby theoutputofthe LM7701 and won ’t go below 50 mV insteadof the desired0V, causinga non-linearityinthe sensorreading. When usinga 12 bitADC, and a referencevoltageof5 Volt(havingan ADC stepsizeofapproximate1.2mV), theoutputsaturationresultsina lossofthelower40 quantizationlevelsoftheADCs dynamicrange. Two-Stage,SingleSupply True Zero Amplifier Thissensorapplicationproducesa DC signal,amplifiedby a two cascadedop amps, havinga singlesupply.The outputvoltageofthesecond op amp isconvertedtothedigitaldomain.Figure30 shows thebasicsetupofthis application. Figure30. Sensor withDC Output and a 2-Stage,SingleSupply Op Amp. The sensorgeneratesa DC outputsignal.In thiscase,a DC coupled,2-stageamplifierisused.The output voltageswing of the second op amp shouldme matched to the inputvoltagerange of the Analog to Digital Converter(ADC).For thehighsideoftherangethiscan be done by adjustingthegainoftheop amp. However, thelowsideoftherangecan’tbe adjustedand isaffectedby theoutputdriveoftheop amp. Example: Assume; the outputvoltagerange of the sensor is0 to 90 mV. The availableop amp isa LMP7702 (Dual LMP7701 op amp) thatcan be used forA1 and A2.The op amp isusinga 0/+5V supplyvoltage,havingan output driveof50mV frombothrails.Thisresultsinan outputrangeof50 mV to4.95Vforeach individualamplifier. Letchoose two resistorsvaluesforR G1 and R F1 thatresultina gainof10x forthefirststage(A1)and a gainof 5x forthesecond stage(A2)The outputoftheA2 intheLMP7702 shouldswingfrom0V to4.5Volt.Thisswingis limitedby the2 differentfactors: 1. The highvoltageswing isno problem;however the low voltageswing islimitedby the outputsaturation voltageofA2 fromtheLM7702 and won ’tgo below50mV insteadofthedesired0V. 2. Anothereffecthas more impact.The outputsaturationvoltageofthefirststagewillcause an offsetforthe inputofthesecond stage.ThisoffsetofA1 isamplifiedby thegainofthesecond stage(10xinthisexample), resultinginan outputoffsetvoltageof500mV. Thisissignificantlymore thatthe50 mV (VDSAT )ofA2. When usinga 12 bitADC, and a referencevoltageof5 Volt(havingan ADC stepsizeofapproximate1.2mV), theoutputsaturationresultsina lossofthelower400 quantizationlevelsoftheADCs dynamic range.Thiswill cause a majornon-linearityinthesensorreading. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 13 ProductFolderLinks:LM7705

SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com Dual Supply,True Zero Amplifiers The limitationsoftheoutputstageoftheop amp, as indicatedinbothexamples,can be omittedby usinga dual supplyop amp. The outputstage of the used op amp can then stillswing from 50 mV of the supplyrails. However,thefunctionaloutputrangeoftheop amp isnow fromgroundleveltoa valuenearthepositivesupply rail.Figure31 shows theoutputdriveofan amplifierina truezerooutputvoltageapplication. Figure31. Amplifieroutputdrivewitha dualsupply Disadvantagesofthissolutionare:

The usage ofa dualsupplyinsteadofa simplesinglesupplyismore expensive.
A dualsupplyvoltagefortheop amps requirespartsthatcan handlea largeroperatingrangeforthesupply voltage.Iftheop amps used inthecurrentsolutioncan’thandlethis,a redesigncan be required. A bettersolutionistouse theLM7705. Thislow noisenegativebiasgeneratorhas some majoradvantageswith respecttoa dualsupplysolution:
Operateswithonlya singlepositivesupply,and isthereforea much cheapersolution.
The LM7705 generatesa negativesupplyvoltageofonly−0.23V.Thisismore thanenough tocreatea True- zerooutputformost op amps.
Inmany applications,this“small” extensionofthesupplyvoltagerangecan be withintheabs max ratingfor many op amps, so an expensiveredesignisnotnecessary. In the next sectiona typicalamplifierapplicationwillbe evaluated.The performanceof an amplifierwillbe measured in a singlesupplyconfiguration.The resultswillbe compared withan amplifierusinga LM7705 supplyinga negativevoltagetothebiaspin. TYPICAL AMPLIFIER APPLICATION Thissectionshows themeasurement resultsofa truezerooutputamplifierapplicationwithan analogtodigital converter(ADC) used as back-end.The biasingoftheop amp can be done intwo ways:
A singlesupplyconfiguration
A singlesupplyincombinationwiththeLM7705, extendingthenegativesupplyfromgroundleveltoa fixed- 0.23Voltage. Basic Setup The basicsetupofthistruezerooutputamplifierisgiveninFigure32.The LMP7701 op amp isconfiguredas a voltagefollowerto demonstratethe outputlimitation,due to the saturationof the outputstage.The negative power supplypinoftheop amp can be connectedtogroundlevelortotheoutputofthenegativebiasgenerator, todemonstratetheVDSAT effectattheoutputvoltagerange.

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www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 Figure32. TypicalTrue Zero Output VoltageApplicationwith/withoutLM7705 The outputvoltageoftheLMP7701 isconvertedtothedigitaldomain usingan ADC122S021. Thisisan 12 bit analogto digitalconverterwitha serialdata output.Data processingand graphicaldisplayingisdone witha computer.The negativepower supplypinoftheop amp can be connectedtogroundlevelortotheoutputofthe negativebiasgenerator,todemonstratetheeffectattheoutputvoltagerangeoftheop amp. The key specificationsoftheused components aregiveninthenextpartofthesection. Supply Voltage/ReferenceVoltage Supplyvoltage +5V ADC VoltageReference +5V LMP7701 VDSAT (typical) 18 mV VDSAT (overtemperature) 50 mV LM7705 Outputvoltageripple 4 mV PP Outputvoltagenoise 10 mV PP ADC Type ADC122S021 Resolution 12 bit Quantizationlevel 5V/4096= 1.2mV Measurement Results The outputvoltagerangeoftheLMP7701 has been measured,especiallytherangetogroundlevel.A smallDC signal,witha voltageswing of 50 mV PP isappliedto the input.The digitizedoutputvoltageof the op amp is measured overa giventimeperiod,when itsnegativesupplypinisconnectedtoground levelor connectedto theoutputoftheLM7705. Figure33A and Figure33B show thedigitizedoutputvoltageoftheLMP7701 op amp. Copyright© 2008–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 15 ProductFolderLinks:LM7705

TIME (SAMPLES) DIGITIZED OUTPUT VOLTAGE (\`V) 0.050 0.040 0.030 0.020 0.010 0.000 0 80 160 240 320 400 VDSAT TIME (SAMPLES) DIGITIZED OUTPUT VOLTAGE (V) 0.050 0.040 0.030 0.020 0.010 0.000 0 80 160 240 320 400 LM7705 SNVS420B –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com A B Figure33. DigitizedOutput Voltagewithout(A)and with(B)LM7705 Figure33A shows thedigitizedoutputvoltageoftheop amp when itsnegativesupplypinisconnectedtoground level.The outputoftheamplifiersaturatesata levelof14 mv (thisisinlinewiththetypicalvalueof18 mV given inthe datasheet)The graph shows some fluctuations(1 bitquantizationerror).Figure33B show the digitized outputvoltageoftheop amp when itsnegativesupplypinisconnectedtotheoutputoftheLM7705. Again,the graph shows some 1 bitquantizationerrorscaused by thevoltagerippleand outputnoise.Inthiscase theop amps outputlevelcan reachthetruezerooutputlevel. The graphsinFigure33 show that:

Witha singlesupply,theoutputoftheamplifierislimitedby theVDSAT oftheoutputstage.
The amplifiercan be used as a truezerooutputusinga LM7705.
The quantizationerrorofthedigitizedoutputvoltageiscaused by thenoiseand thevoltageripple.
UsingtheLM7705 does notincreasethequantizationerrorinthissetup. DESIGN RECOMMENDATIONS The LM7705 is a switchedcapacitorvoltageinverter.This means thatcharge is transferredfrom different externalcapacitors,to generatea negativevoltage.For thisreason the partis very sensitiveforcontact resistancebetween thepackage and externalcapacitors.It’s alsorecommended touse low ESR capacitorsfor C FLY ,C RES and C OUT incombinationwithshorttraces. To preventlargevariationsat the VDD pinof the package itisrecommended to add a decouplecapacitoras closetothepinas possible. The outputvoltagenoisecan be suppressedusinga smallRF capacitor,willa valueofe.g.100 nF.

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www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013

REVISION HISTORY

www.ti.com 11-Apr-2013 Addendum-Page 1 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Drawing Pins Package Qty Eco Plan (2) Lead/Ball Finish MSL Peak Temp (3) Op Temp (°C) Top-Side Markings (4) Samples LM7705MM/NOPB ACTIVE VSSOP DGK 8 1000 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 F26A LM7705MME/NOPB ACTIVE VSSOP DGK 8 250 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 F26A LM7705MMX/NOPB ACTIVE VSSOP DGK 8 3500 Green (RoHS & no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 F26A (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. TBD: The Pb-Free/Green conversion plan has not been defined. Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above. Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material) (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. (4) Multiple Top-Side Markings will be inside parentheses. Only one Top-Side Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation of the previous line and the two combined represent the entire Top-Side Marking for that device. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release. In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.

*All dimensions are nominal Device Package Type Package Drawing Pins SPQ Reel Diameter (mm) Reel Width W1 (mm) (mm) (mm) (mm) (mm) W (mm) Pin1 Quadrant PACKAGE MATERIALS INFORMATION www.ti.com 26-Mar-2013 Pack Materials-Page 1

*All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM7705MM/NOPB VSSOP DGK 8 1000 210.0 185.0 35.0 LM7705MME/NOPB VSSOP DGK 8 250 210.0 185.0 35.0 LM7705MMX/NOPB VSSOP DGK 8 3500 367.0 367.0 35.0 PACKAGE MATERIALS INFORMATION www.ti.com 26-Mar-2013 Pack Materials-Page 2

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Summary

LM7705 Low Noise Negative Bias Generator

Description

LM7705 Low Noise Negative Bias Generator

Source title LM7705MME/NOPB Datasheet(PDF) - Texas Instruments

Read LM7705MME/NOPB online.

LM7705 TI | Alldatasheet

Document overview

Technical content

2 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

1 C F+ C FLY PositiveCapacitorConnection

2 VSS Power Ground

3 SD Shutdown Pin

4 VDD PositiveSupplyVoltage

5 VSS Power Ground

6 VOUT OutputVoltage

7 C RES ReserveCapacitorConnection

8 C F- C FLY NegativeCapacitorConnection

4 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

6 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

8 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

APPLICATION INFORMATION

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12 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

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16 SubmitDocumentationFeedback Copyright© 2008–2013,Texas InstrumentsIncorporated

REVISION HISTORY

LM7705MME/NOPB specifications and overview.

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